1. Field of the Invention
The present invention generally relates to a power converting circuit and a controller thereof, and more particularly, to a power converting circuit having latching and protecting functions and the controller thereof.
2. Description of Related Art
Current power supplies are mainly classified into linear power supplies (LPS) and switching power supplies (SPS). The LPS has a simple circuit, small ripples, and less electromagnetic interference (EMI). However, electric devices in the circuit are large, so that volume of the circuit is large, weight thereof is heavy, and further, conversion efficiency thereof is low. On the contrary, the SPS has a more complex circuit, larger ripples, and more EMI, but the SPS is still the mainstream in the market of power supply since it has higher conversion efficiency and less power consumption while idling.
FIG. 1 is a schematic circuit of a SPS configured to drive a lamp in a related art. Referring to FIG. 1, The SPS includes an initial resistor R, an initial capacitor C2, a Zener diode Z, a controller CON, a high-side driving capacitor C1, a high-side driving transformer T1, a high-side transistor switch M1, a low-side transistor switch M2, a diode D, an output capacitor C3, and a transformer T2. The SPS is configured to convert a DC input voltage VIN to an AC output voltage VOUT to drive a lamp LAMP.
When the DC input voltage is inputted, a current is supplied to the initial capacitor C2 through the initial resistor R, so that a voltage drop across the initial capacitor C2 is raised until it is equal to the breakdown voltage of the Zener Diode Z. The initial capacitor C2 generates a driving voltage VDD to supply electric power required for operating to the controller CON. When the driving voltage VDD is higher than a start voltage of the controller CON, the controller CON starts, so as to generate control signals to control the high-side transistor switch M1 and the low-side transistor switch M2. Wherein, the controller CON raises a voltage level of the control signal up to a suitable voltage level to control the high-side transistor switch M1 through the high-side driving capacitor C1 and the high-side driving transformer T1. By switching the high-side transistor switch M1 and the low-side transistor switch M2, the electric power of the DC input voltage VIN is transmitted to an output terminal to generate the AC output voltage VOUT to drive the lamp LAMP. The transformer T2 is coupled to the AC output voltage VOUT, and transmits electric power, rectified by the diode D, to the initial capacitor C2.
The initial capacitor C2 gradually stores the electric power because the electric power transmitted through the initial resistor R is more than the electric power consumed by the controller CON before the controller CON starts. After the controller CON started, the electric power through the transformer T2 and the diode D is also supplied to the controller CON. Accordingly, the initial resistor R having a relatively large resistance is used to lower power consumption by the initial resistor R. However, when an abnormal event occurs in the circuit, the DC input voltage VIN can not supply the electric power to the AC output voltage VOUT, so that the transformer T2 and the diode D can not supply the electric power. Moreover, the electric power transmitted through the initial resistor R is not enough to hold all of the electric power required by the controller CON while normally operating, so that the operation of the controller CON may fail.
FIG. 3A illustrates a schematic signal waveform of the SPS configured to drive the lamp in the related art while the circuit stays in the abnormal state. Referring to FIG. 3A, When the driving voltage VDD is higher than the start voltage UVLO, the SPS starts to operate. Since an oscillator and a control circuit inside the controller CON start to operate, the current consumed thereby is much more than the current supplied by the DC input voltage VIN through the resistor R. Accordingly, the driving voltage may start to fall down. When the circuit operates at a normal state, the controller CON outputs signals to switch the high-side transistor switch M1 and the low-side transistor switch M2, so that the AC output voltage VOUT is raised, and the electric power is supplied to the driving voltage VDD through the transformer T2 and the diode D. However, when the abnormal event occurs in the circuit, the controller CON stops switching the high-side transistor switch M1 and the low-side transistor switch M2, so that the AC output voltage VOUT is lowered and can not supply the electric power to the driving voltage VDD. As a result, the driving voltage VDD may start to fall down. When the driving voltage VDD has become lower than a voltage range which the controller CON can tolerate, the controller CON stops operating and further consuming power. Accordingly, the driving voltage VDD is raised again until it is higher than the start voltage UVLO, so that the controller CON re-starts. The above-described cycle is repeated until the abnormal event is eliminated. Furthermore, in order to avoid the temporary abnormal event resulting in lamp not lighting, the controller CON may try to turn on the lamp continuously when lamp is not lighting in the related art. In the process, not only life-span of the lamp is shortened due to limitation of start cycles thereof, but also users may get an electric shock while lamp replacing since the users forget to turn off the power source. Accordingly, since the lamp driving circuit may re-start the lamp in the SPS of the related art, not only the life-span of the lamp is shortened, but also using it may be dangerous to the users.